1. Field of the Invention
This invention relates to films useful as encapsulants for photovoltaic (solar) cells. This invention particularly relates to films comprising ethylene acid copolymer ionomers useful as encapsulants for photovoltaic cells.
2. Description of the Related Art
In applications where good optical properties are important, it can be a requirement to have a film that is substantially transparent to light. Films useful in applications where optical properties are important may not have other physical or mechanical properties—such as, for example modulus—suitable for use in the desired field of use.
Blends of ionomers have found particular application in golf ball covers. For example U.S. Pat. No. 5,587,430 to DuPont describes an ionomer composition for improved high temperature performance.
U.S. Pat. No. 5,567,772 to DuPont describes an ionomer blend comprising a polymer that is an ethylene/acid copolymer ionomer which is a partially neutralized copolymer of ethylene and methacrylic acid or acrylic acid.
Golf ball covers have been obtained from blends of high acid ionomers with other ionomers. For example U.S. Pat. No. 5,873,796 to Acushnet Company claims a golf ball comprising a core and at least one cover layer comprising a blend of a high acid ionomer formed from an acid copolymer having an acid content of 19 weight percent; and an ionomer formed from an acid copolymer having an acid content of 15 weight percent.
Blends of high and low modulus ionomers for use in a golf ball have been described in U.S. Pat. No. 5,415,937.
U.S. Pat. No. 4,884,814 to Spalding teaches blending a hard ionomer resin with a soft ionomer resin to produce a golf ball cover.
U.S. Pat. No. 5,120,791 to Lisco describes a composition for use in a golf ball said to have enhanced carrying distance.
Other patents of interest that demonstrate the current state of the art in blending of ionomers are U.S. Pat. Nos. 5,688,869, 5,691,418, and 5,328,959.
The teaching of the golf ball related art describes blended compositions suitable for use in a golf ball cover or other parts of a golf ball. It is of note, however, that transparency and haze (optical properties) are not properties that are considered critical or of any influence to golf ball performance or acceptance.
Blending ionomers having a modulus that is acceptable for an encapsulant application with other ionomers having acceptable optical properties can result in optical properties and modulus that are intermediate between the properties of the components. This can be an unsatisfactory result where improved optical properties or modulus are desirable.
It would therefore be desirable to provide an ionomer film that incorporates both the physical and optical properties desirable for use in applications where both physical properties and optical properties of a film are important.